1. Field of the Invention
The present invention relates to a three-dimensional measurement apparatus, and more particularly to an apparatus for use in a solder paste printing machine and the like for an electronic component surface mounting system.
2. Description of the Related Art
On a printed circuit board to which electronic components are mounted, a solder paste pattern is generally printed for forming a conductor between the components and a wired pattern on the board. For measuring the shapes of the printed paste, three-dimensional measurement using the "light-section method" is widely applied. In the light-section method, an object (for example, solder paste) is irradiated by a stripe of light so as to section the surface thereof, and measure the shape or unevenness of the surface thereof from intersected lines on the surface. One example is shown in FIG. 8.
Referring to FIG. 8, laser light emitted from a laser diode 21 is focused through an aspheric focusing lens 22 on the vicinity of an object 28 as a beam spot of 0.2 mm in diameter. The laser light passing through the focusing lens 22 is reflected by a galvano mirror 23 disposed in the midst of light path, passes through a semi-transparent mirror 24, and is projected on the object 28. As a galvano mirror 23, driven by a driver (not shown) oscillates as indicated by an arrow in FIG. 8, the beam spot is scanned substantially in a line on the surface of object 28, thereby forming a stripe of light necessary for the light-section method.
The scanned light is also reflected off the semi-transparent mirror 24 and directed to a position sensitive device (PSD) 25, which detects a position in the scanning direction (X-axis) based on the rotating angle of the mirror 23. Simultaneously, the reflected light from the object surface is focused on a PSD 27 provided for detecting a position in the vertical direction (Z-axis) through a receiving lens 26 disposed at an angle of between 18 and 30 degrees with the projected light axis. The spot position focused on the PSD 27 changes depending on the height of the object and can be taken out as a voltage change, thereby providing a measurement of the unevenness (position in the Z-axis) of the surface of the object 28. With the repetition of abovementioned measuring method according to the movement of the whole measurement apparatus in a direction (Y-axis) perpendicular to the optical scanning direction, three-dimensional positions of the object 28 are measured.
In the foregoing three-dimensional measurement method, PSDs are used for detecting scanned positions. However, since PSDs do not have good voltage linearity characteristics, the detected positions must be corrected based on the calculation of characteristics of each PSD to be used. Another problem with the conventional PSDs is that because a change of the center of gravity of the focused spot on PSD is taken out when measuring height by PSDs, spot luminance related data is not obtainable. Also, PSD characteristics change depending on temperature, so it is difficult to measure accurate height.
Another problem arises in the conventional three-dimensional measurement method. Since the optical path length between the aspheric focusing lens and the object is long, which means a small numerical aperture (N.A.), and the laser light can not be focused to a small diameter, it is difficult to obtain a highly accurate measurement result.
Still another problem arises in the conventional three-dimensional measurement method. Since the whole measurement apparatus must be moved in a direction perpendicular to the longitudinal direction of the stripe of light every time the stripe of light is cast for the measurement of the whole surface of the object, a driving device for movement of the apparatus would be on a large scale, and thereby vibration would occur during movement. Consequently, accurate measurement is difficult.
Thus, as demonstrated by the foregoing problems, there exists a need for a three-dimensional measurement apparatus for measuring three-dimensional positions of an object with high accuracy and high speed.